An investigation is being made of the kinetics of branch migration of the junction present in the figure 8 configuration of the replicative form of phage G4 DNA. The figure 8 is converted to an X form by treatment with EcoR1 restriction endonuclease. Branch migration leads to the formation of two linear monomers from a dimeric X form. The reaction is followed by quantitating the formation of monomers. Studies are being made or are planned on the effect on the rate of branch migration of temperature, ionic strength, ethidium bromide, protein and small molecule modifiers and supercoiling. Figure 8s isolated from G4 replicative form have been used in previous work. They are now being made in larger quantity by annealing dimer length single strands of phage G4 replicative form with G4 viral DNA. This permits isolation in microgram quantities of DNA containing more than 80% figure 8s from a CsCl-propidium iodide gradient in which annealed figure 8s behave as half-supercoiled molecules. Genetic experiments are underway and are planned that together with information on the characteristics of branch migration are being used to test a proposed mechanism for recombination in single-stranded DNA phages. The genetic investigations include work on single-burst analysis, recombination between phiX174 and G4 and the behavior of genetically labeled figure 8s in spheroplasts. Investigation of the kinetics of renaturation of denatured covalently-closed, circular DNA is being continued. The emphasis is on the effect of the conditions of denaturation on the rate of renaturation.